Substantial volumes of mixtures, predominantly of sulfuric acid and nitric acid containing small percentages of nitric acid, are obtained from various commercially important chemical operations. The economic utilization of such mixtures is a continuing problem of the chemical industry. The actual concentrations of nitric acid and sulfuric acid in such mixtures vary. Many of these mixtures result from the treatment of organic compounds with nitrating mixtures which are blends of concentrated nitric acid and concentrated sulfuric acid. Nitrating mixtures of this kind are termed mixed acid. Standard mixed acids are articles of commerce. As a result of the treatment, the organic compound is converted to a nitro organic compound. In addition, there is formed a residual acid solution composed of the unconsumed nitric acid, all of the sulfuric acid originally present in the mixture, and the water which is the other principal product of the nitration reaction. The nitric acid concentration must be sufficient to obtain the desired nitrated product at the end of the reaction. This residual acid solution is termed spent nitration acid since it no longer has any nitration capability for the desired nitration reaction. It also always contains small concentrations of the desired product, by-products and other impurities.
Many nitration operations are of such a nature that the spent nitration acid can be economically processed to recover the unconsumed nitric acid at the same time the sulfuric acid is recovered. The recovered sulfuric acid can then be reconcentrated. As a result, both acids can be employed to make additional mixed acid. This processing generally consists of diluting the acid so that the sulfuric acid concentration remains below about 77%, fractionally distilling the nitric acid out of the mixture and recovering it, and subsequently reconcentrating the denitrated sulfuric acid.
Processing spent acid as described above is only economically feasible when very large volumes of spent acid are available and when the spent acid is free from extraneous substances which cause processing problems such as acid darkening, foaming, etc. In addition, the spent nitration acid must be free from substances which would cause the acid to be excessively corrosive to the materials of construction used in the fabrication of the expensive processing equipment needed to carry out the above process. Thus, spent nitration acid from the nitration of fluorine-containing organic compounds are especially difficult to process. They contain small concentrations of fluoride ion which make them corrosive to glass, ceramics, and tantalum, which are the best materials of construction for handling sulfuric acid and nitric acid.
Because of the special circumstances required to economically process spent nitration acid, at one time much spent acid was used in the fertilizer industry to acidulate phosphate rock. This is no longer possible unless special precautions are taken during the acidulation operation to prevent the escape of air polluting gaseous oxides of nitrogen. These form when phosphate rock is contacted by spent acid containing more than about 0.1% of dissolved nitric acid. Consequently, increasingly large amounts of spent acid now have to be treated with lime and the resultant calcium sulfate disposed of as land-fill. These operations are only permitted under circumstances where the drainage is so controlled that it cannot be the cause of water pollution.
It is obvious that any spent acid neutralization process cannot be regarded as satisfactory since it consumes alkali of one type or another and the acid value of the spent acid is lost simultaneously. The problem of the economic utilization of spent acid is becoming increasingly acute as environmental considerations become increasingly important. Various proposals have been advanced to mitigate this problem but none are as satisfactory as the present invention.
Thus, the present invention is concerned with spent nitration acid, and more specifically with the economic removal of its nitric acid content. The removal of the nitric acid makes the residual acid suitable for purposes for which the untreated spent acid could not be used.